The decorating injection molding method has been widely used to decorate resin moldings having a complicated surface such as a three-dimensional curved surface (for example, Patent Documents 1 and 2). The decorating injection molding is a method of decorating the surface of resin moldings by injection molding, in which a decorative sheet having been placed in a mold is united with a molten resin injected into the mold cavity, and is roughly classified into a laminate decoration method and a transfer decoration method according to the difference in the constructions of the decorative sheet to be united with the resin moldings.
In the laminate decoration method, whole layers of a decorative sheet having a base film and a decorative layer formed thereon are unitedly laminated to the surface of resin moldings, while using a decorative laminate sheet as the decorative sheet. In the transfer decoration method, a similar decorative sheet is unitedly laminated to the surface of resin moldings and then only the base film is released with the transfer layer such as a decorative layer being remained on the resin moldings, while using a transfer sheet as the decorative sheet.
An example of the decorating injection molding method, in which a transfer sheet is used as the decorative sheet, is described below with reference to the attached FIGS. 4 and 5. FIGS. 4 and 5 illustrate an example of the molding steps of a decorating injection molding method, in which a decorating injection molding machine 60 has a female mold 70 and a male mold 80 which is disposed oppositely to the female mold 70 at its side. The female mold 70 has a cavity 72 corresponding to the contour of moldings to be produced. Suction holes 74 extending throughout the female mold 70 are further provided so as to open to the cavity 72. The female mold 70 is made reciprocally movable in the directions approaching to and coming apart from the male mold 80 by a reciprocating means 75 such as a cylinder. The male mold 80 has a core 82 which is inserted into the cavity 72 and a gate 84 for injecting a molten resin. Between the female mold 70 and the male mold 80, a reciprocally movable heating plate 90 is optionally disposed, if necessary.
To conduct the decoration simultaneously with the injection molding by the use of the machine 60, a decorative sheet 100 is first disposed oppositely to the female mold 70 at its side. After optionally softening the decorative sheet 100 at appropriate temperatures by the heating plate 90, the opening of the cavity 72 is closed by the decorative sheet 100 which is held between the female mold 70 and the heating plate 90. Then, the cavity 72 is evacuated through the suction holes 74 formed in the female mold 70, while simultaneously supplying a pressurized air, if necessary, onto the decorative sheet 100 through a vent hole formed in the heating plate 90. Both the molds are heated generally at 30 to 50° C.
By such an operation, the decorative sheet 100 is drawn along the inner surface of the cavity 72 and then closely adhered to the cavity 72, as shown in FIG. 4. This step is generally called as a preforming process, in which a softened sheet is usually drawn about 200% at highest. Next, the heating plate 90 is withdrawn and the female mold 70 is allowed to move forward so as to put it together with the male mold 80 as shown in FIG. 5. After clamping the molds, a cavity formed between the female mold 70 and the male mold 80 is filled with a fluidized resin molding compound P by injecting it through the gate 84 formed in the male mold 80, to perform the injection molding.
By such an injection molding, the decorative sheet 100 placed in the female mold 70 and the injected resin are unitedly bonded, and a molding having its outer surface coated with the decorative sheet 100 is obtained by the mold break after the completion of the injection molding. In the subsequent step, only the base film is peeled off from the decorative sheet 100 united to the outer surface of the molding to leave a transfer layer such as a decorative layer on the molding, thereby completing the decoration.
In the decorating injection molding method described above, it is important for producing good moldings that the decorative sheet 100 is stretched along the inner surface of the cavity 72 and then closely adhered to the cavity 72 during the preforming process or the injection of molten resin (moldability), without an unfavorable deformation due to excessive drawing exceeding the extent necessary for allowing the decorative sheet to match the shape of the mold, which can be caused by the action of vacuum and pressure, the pressure of molten resin or the drawing due to shearing stress. This is also true in the case of using a laminate sheet as the decorative sheet, and particularly important in the molding using a deep mold because the decorative sheet is subject to deep drawing.
If the depth of draw forming is large (the stretch ratio of decorative sheet is large), it is required for such a decoration of injection molded articles to decorate the decorative sheet until it reaches the drawn shape corresponding to a draw ratio of 180 to 200% at highest. To meet this requirement, polyvinyl chloride (PVC), acrylonitrile-butadiene-styrene copolymer resin (ABS resin), etc. have been used for the base film of the transfer sheet and laminate sheet because of their good thermoformability. However, the decorative sheet having a base film made of such a resin may be, particularly when subjected to a deep drawing, excessively deformed, fluidized or broken during the vacuum-pressure forming in the preforming process and also by the heat and pressure of the injected molten resin in the injection molding. In addition, the film around the gate for resin injection is drawn by the injected resin, thereby likely to cause the drawn film to bite into the resin molding. Further, such a decorative sheet is poor in the transparency, surface smoothness, coating appearance, etc. Particularly in the case of the decorative sheet having a PVC film, the adhesive strength between the resin molding and the decorative sheet is liable to decrease with time because of the migration of plasticizer.
To remove these problems, there has been disclosed a decorated molding in which the base film of decorative sheet is mainly made of a methyl acrylate-butyl acrylate copolymer, a methyl acrylate-butyl methacrylate copolymer, a methyl methacrylate-butyl acrylate copolymer or a methyl methacrylate-butyl methacrylate copolymer (for example, Patent Document 3). It is further disclosed that a film having a thickness of 50 to 260 μm and a stress at 200% elongation of 20 to 70 kgf/cm2 (196 to 686 N/cm2) at 100° C. is preferable as the base film for the decorative sheet. However, the proposed technique relates to a decorative sheet having a base film made of a (meth)acrylic copolymer. If such a decorative sheet is used as the transfer sheet, the base film is hardly peeled off (defective transfer), because the adhesion between the base film and the transfer layer (decorative layer) made of a printing ink layer is excessively high. If used as the laminate sheet, the resistance to peel-off is in turn advantageous, but, the base film is insufficient in solvent resistance and stain resistance which are required for the surface protective layer.    [Patent Document 1] JP 50-19132B    [Patent Document 2] JP 61-17255B    [Patent Document 3] JP 8-276544A